1. Field of the Invention
The present invention relates to an optical disc apparatus for reading information recorded on an optical disc medium such as a CD, a DVD, or a Blu-ray disc (trademark), a method of controlling the optical disc apparatus, and an information storage medium.
2. Description of the Related Art
In recent years, various types of optical disc media have been used as information recording media. In general, the optical disc medium has a structure in which a plurality of layers such as a data recording layer for recording information thereon and a protective layer for protecting the data recording layer are laminated. Nowadays, in particular, in order to improve a recording density, there also exists an optical disc medium including a plurality of the data recording layers, which enables the information to be recorded on or to be read from each of the data recording layers. In order to read the information recorded on such an optical disc medium, an optical disc apparatus is used. The optical disc apparatus includes an optical pickup for irradiating the optical disc medium with light to detect light reflected from the optical disc medium.
When the optical disc apparatus reads the information recorded on the optical disc medium, it is necessary to adjust a focus of an objective lens included in the optical pickup to be on a signal surface (surface of the data recording layer) of the optical disc medium. Therefore, when the information is read, the optical disc apparatus performs a focus detection operation for detecting a focus state where the focus of the objective lens is on the signal surface based on an output signal from the optical pickup (for example, see Japanese Patent Application Laid-open No. 2007-052896).
The state where the focus of the objective lens is on the signal surface can be detected, for example, in the following manner. Specifically, in the case where the objective lens is gradually brought closer to a surface of the optical disc medium, a pull-in signal (PI signal) indicating a total level of the output signals from the optical pickup exhibits a peak when the focus of the objective lens is on the signal surface. In addition, a focus error signal (FE signal) indicating a deviation of a focal position of the objective lens with respect to the signal surface has a predetermined waveform in the vicinity of the state where the focus of the objective lens is on the signal surface. Therefore, the focus state can be detected by detecting a waveform of the FE signal, which satisfies a certain condition within the range where a level of the PI signal is equal to or higher than a given threshold value. For such a purpose, a circuit for outputting a comparison signal (focus OK (FOK) signal) indicating whether or not the level of the PI signal is a given threshold value Th or higher is provided. The FOK signal is, for example, a binary signal which has a High level when the level of the PI signal is equal to or higher than the threshold value Th and has a Low level when the level of the PI signal is less than the threshold value Th. The comparison circuit for outputting such an FOK signal can be easily realized by a simple circuit such as a comparator.